This invention relates to compounds which function to increase the state (degree) of vulcanization when used during the sulfur vulcanization of rubbers by donating sulfur to the system. This invention also relates to compounds which provide a vulcanizable polymer with balanced processing and vulcanization characteristics. In addition, it relates to compounds which retard vulcanization during the processing of vulcanizable rubbery compositions. It also relates to compounds which function as activators, that is, secondary accelerators in sulfur type vulcanization systems. It also relates to processes for increasing the state of vulcanization of sulfur vulcanizable rubbery compositions and either increasing scorch delay periods and/or increasing vulcanization rates. It also relates to the vulcanized products resulting therefrom.
The physical properties of a vulcanized composition are related to its state of vulcanization. Often, as the state of vulcanization is increased, certain physical properties are improved. Rubber additives such as sulfur donors can be used therefore to increase the state of vulcanization or to permit the use of lesser amounts of free sulfur.
Scorching during the processing of rubber is due to the premature or insipient vulcanization which can occur during any of the steps involved in the processing of the rubber prior to the final vulcanization step or during storage between said processing steps. Whereas a properly compounded unscorched rubber formulation can be die extruded or sheeted smoothly from a calender without lumping, a scorched material often becomes wavy or lumpy after sheeting and must be discarded. It is therefore desirable that rubber additives be used which reduce scorching. Such compounds are commonly referred to as retarders.
It is often desirable to increase the rate at which rubbery compositions are vulcanized. Sulfur vulcanizable rubbery compositions containing free sulfur are made to vulcanize more rapidly by the addition of an accelerator compound. Often the accelerator compound is referred to as a primary accelerator and is used in combination with another accelerator called an activator or secondary accelerator which further increases the vulcanization rate.
It is an object of this invention to provide sulfur donors which will increase the state of vulcanization of vulcanized rubbery polymers as well as compounds which are secondary accelerators (activators) and/or retarders (scorch inhibitors). Another object of the present invention is to provide processes which will improve the scorch resistance and rate of vulcanization of vulcanizable rubbery polymers and also the state of vulcanization of sulfur vulcanized rubbers.